circuits

A pathway for electrons to flow. Electric Circuits A pathway for electrons to flow.

                             Open Circuits

Closed Circuits

Dry Cell How do batteries work? Batteries have three parts, an anode (-), a cathode (+), and the electrolyte. The cathode and anode (the positive and negative sides at either end of a traditional battery) are hooked up to an electrical circuit.                                   Dry Cell The chemical reactions in the battery causes a build up of electrons at the anode. This results in an electrical difference between the anode and the cathode. You can think of this difference as an unstable build-up of the electrons. The electrons wants to rearrange themselves to get rid of this difference. But they do this in a certain way. Electrons repel each other and try to go to a place with fewer electrons.

Series Circuit The current has only one path to follow.

CREATE your own circuits

Parallel Circuits Two or more branches for the current to flow.

In a series circuit , the current through each of the components is the same, and the voltage across the components is the sum of the voltages across each component. In a parallel circuit, the voltage across each of the components is the same, and the total current is the sum of the currents through each component.

a. What is the voltage across each resistor. b a. What is the voltage across each resistor? b. What is the current in each branch? c. What is the total current provided by the battery? The voltage is equal across all components in a parallel circuit. (Therefore, the voltage across R1 is equal to the voltage across R2 which is equal to the voltage across the battery.) The total current in the circuit is equal the sum of all individual branch currents.

Electric Power The rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt Joule's Law

What is a kilowatt hour? How long you run an appliance. How much energy is used? Energy used = Power (kW) x Time (hrs) E= P x t To find cost: Ex: 10¢ per kilowatt hour E x $

105 V are used to power an appliance that needs 15. 0 amps 105 V are used to power an appliance that needs 15.0 amps. What is the power used? 1575 W =1.575kW How much energy is used when this appliance is used for 30.0 days- 24hrs a day?30x24=720hrs x 1.575kW= 1134 kW -hr If the power company charges 8¢/Kw-h, what is the cost of the energy above. 1134kW-hr x .08= $90.72 An electric refrigerator rated 400 W operates 8 hour/day. What is the cost of energy to operate it for 30 days at 8¢/Kw-h? $96.00

How a Lightbulb Works

a. What is the total voltage across the bulbs. b a. What is the total voltage across the bulbs? b. What is the total resistance of the circuit? c. What is the current in the circuit? D. What happens to the brightness of each bulb in a series circuit as additional bulbs are added? Why?

symbols to represent each piece. Circuit Diagrams Drawing realistic pictures of circuits can be very difficult. For this reason, we have common symbols to represent each piece. Resistor Battery Wire *Note: Circuit diagrams do not show where each part is 
physically located.

Circuit Diagrams Diagram Draw a simple circuit that has a 9 V battery with a 3 Ω resistor across its terminals. What is the magnitude and direction of the current? R = 3Ω V = 9 V I I = 3A Diagram Conventional current flows from the positive 
terminal to the negative terminal.

Circuit Diagrams There are two ways to add a second resistor to the circuit. Series Parallel R1 R2 V R1 R2 V All charges must move 
through both resistors to get 
to the negative terminal. Charges pass through either R1or R2 but not both.

Circuit Diagrams Are the following sets of resistors in series or parallel? R1 R2 V R1 R2 V Series Parallel The "test" is to trace the shortest route around the circuit. 
The resistors found on the same route are in series; those 
not found on the same route are in parallel to those that were. Answer

Equivalent Resistance Resistors and voltage from batteries determine the 
current. Circuits can be redrawn as if there were only a single 
resistor and battery.By reducing the circuit this way, the circuit becomes easier to study. The process of reducing the resistors in a circuit is 
called finding the equivalent resistance (Req). R1 R2 V

Series Circuits: Equivalent Resistance What happens to the current in 
the circuit to the right? What happens to the voltage as it 
moves around the circuit? Answer The current passing through 
all parts of a series circuit is the 
same. For example: I = I1 = I2 R1 R2 V Answer The sum of the voltage drops 
across each of the resistors in a 
series circuit equals the voltage 
of the battery. For example: V = V1 + V2

Series Circuits: Equivalent Resistance If V = V1 + V2 + V3 + ...   IR = I1R1 + I2R2 + I3R3 IR = IR1 + IR2 + IR3 Req = R1 + R2 + R3 + ... substitute Ohm's Law solved for V is: V = IR  but since current (I) is the same everywhere in a series circuit, I = I1 = I2 = I3  Now divide by I To find the equivalent resistance (Req) of a series circuit, add the resistance of all the resistors.If you add more resistors to a series circuit, what happens to the resistance?

19 What is the equivalent 
resistance in this circuit? R1 = 5Ω R2 = 3Ω V = 9 V answer Resistors in series:

20 What is the total current at 
any spot in the circuit? R1 = 5Ω R2 = 3Ω V = 9 V answer Resistors in series:

Net Current/equal everywhere: 21 What is the voltage drop 
across R1? answer Resistors in series: Net Current/equal everywhere: Voltage Drop across R1: R1 = 5Ω R2 = 3Ω V = 9 V

Net Current/equal everywhere: 22 What is the voltage drop 
across R2? answer Resistors in series: Net Current/equal everywhere: Voltage Drop across R2: R1 = 5Ω R2 = 3Ω V = 9 V hint: A good way to check your work is to see if the voltage drop across all resistors equals the total voltage in the circuit.

Net Current/equal everywhere: 23 How much power is used 
by R1? answer Net Current/equal everywhere: Voltage Drop across R1: Power used by R1: R1 = 5Ω R2 = 3Ω V = 9 V

24 What is the equivalent 
resistance in this circuit? R1 = 10Ω R2 = 20Ω V = 9 V answer Resistors in series:

25 What is the total current at 
any spot in the circuit? answer Resistors in series: R1 = 10Ω R2 = 20Ω V = 9 V

Net Current/equal everywhere: 26 What is the voltage drop 
across R1? answer Resistors in series: Net Current/equal everywhere: Voltage Drop across R1: R1 = 10Ω R2 = 20Ω V = 9 V

Net Current/equal everywhere: 27 What is the voltage drop 
across R2? answer Resistors in series: Net Current/equal everywhere: Voltage Drop across R2: R1 = 10Ω R2 = 20Ω V = 9 V

Net Current/equal everywhere: 28 How much power is used 
by R1? answer Net Current/equal everywhere: Voltage Drop across R1: Power used by R1: R1 = 10Ω R2 = 20Ω V = 9 V

Net Current/equal everywhere: 29 How much power is used by 
R2? answer Net Current/equal everywhere: Voltage Drop across R2: Power used by R2: R1 = 10Ω R2 = 20Ω V = 9 V

Parallel Circuits: Equivalent Resistance What happens to the current in 
the circuit to the right? What happens to the voltage as it 
moves around the circuit? Answer The sum of the currents 
through each of the resistors in a 
parallel circuit equals the current 
of the battery.For example: I = I1 + I2 R1 R2 V Answer The voltage across all the resistors in a parallel circuit is the same. For example: V = V1 = V2

Parallel Circuits: Equivalent Resistance If I = I1 + I2 + I3 V1 R1 V R V3 R3 V2 R2 + = 1 ( Req  Rewrite Ohm's Law for I and substitute for each resistor Also, since V = V1 = V2 = V3 so we can substitute V for any other voltage Voltage is a common factor, so factor it out! Divide by V to eliminate voltage from the equation. If you add more resistors in parallel, what will happen to the resistance of the circuit?

V = 18V 30 What is the equivalent resistance in the circuit? R1 = 3Ω answer 30 What is the equivalent 
resistance in the circuit? R1 = 3Ω R2 = 6Ω V = 18V

18 V V = 18V 31 What is the voltage at any spot in the circuit? answer 18 V

32 What is the current 
through R1? answer R1 = 3Ω R2 = 6Ω V = 18V

33 What is the current 
through R2? answer R1 = 3Ω R2 = 6Ω V = 18V

Power used by R1: V = 18V 34 What is the power used by R1? R1 = 3Ω answer Power used by R1: R1 = 3Ω R2 = 6Ω V = 18V

Power used by R2: V = 18V 35 What is the power used by R2? R1 = 3Ω answer Power used by R2: R1 = 3Ω R2 = 6Ω V = 18V

Series vs parallel Series: Sum of voltage= V1 +V2 Equal Current = I =I Different Power = IV1 Power = IV2 Parallel: Equal voltage= V1=V2 Sum of Current = I1 + I2 Different Power = I1V Power = I2V

Measurement Return to Table of Contents

Voltmeter Voltage is measured with a voltmeter. Voltmeters are connected in parallel and measure the difference in potential between two points. Since circuits in parallel have the same voltage, and a voltmeter has 
very high resistance, very little current passes through it. This means that it has little effect on the circuit.

Ammeter Current is measured using an ammeter. Ammeters are placed in series 
with a circuit. In order to not interfere with the current, the ammeter has a very low resistance.

Multimeter Although there are separate items to 
measure current and voltage, there 
are devices that can measure both 
(one at a time). These devices are called 
multimeters.Multimeters can also measure resistance. Click here for a PhET simulation on circuits

LAB WORK